Apparatus and method for inspecting an electrolytic capacitor in an inverter circuit containing transistor switching elements

ABSTRACT

An apparatus for inspecting an electric component in an inverter circuit has a DC power supply for supplying a direct current to a given position in an inverter circuit, a voltage detector for detecting a voltage at a given position in the inverter circuit, a current detector for detecting a current flowing at a given position in the inverter circuit, a switching circuit for changing positions at which the direct current is supplied from the DC power supply, positions at which the voltage is detected by the voltage detector, and positions at which the current is detected by the current detector, and a controller for outputting a switching signal to the switching circuit. The switching circuit is controlled by the controller to charge an electrolytic capacitor in an inverter circuit with a current from the DC power supply. The electrolytic capacitor is determined as to its quality by determining whether the calculated electrolytic capacitance of the electrolytic capacitor falls within a preset range or not. Each of the transistors of the inverter circuit is determined as to its quality by determining whether an V CE  -I C  curve thereof falls in a preset range or not. The electrolytic capacitor, the transistors, and also diodes connected across the transistors can be determined as to whether they are acceptable or not while they are being connected in the inverter circuit.

This application is a divisional of application Ser. No. 08/610,754,filed on Mar. 4, 1996, now U.S. Pat. No. 5,694,051, which was adivisional of Ser. No. 08/337,479, filed on Nov. 8, 1994 (U.S. Pat. No.5,497,095--Issued Mar. 5, 1996), the entire contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for inspecting an electriccomponent in an inverter circuit, and more particularly to an inspectionapparatus for inspecting an electric component in an inverter circuitwhich converts a direct current into an alternating current while theelectric component is being connected in an inverter circuit.

2. Description of the Related Art

Inverters have heretofore been used for turning on and off transistorsbased on control pulses supplied from a controller to convert a directcurrent into an alternating current for energizing a load such as awelding machine, an AC motor, or the like.

One typical inverter circuit comprises an electrolytic capacitor forsmoothing a voltage, a plurality of transistors, and a plurality ofdiodes coupled in inverse-parallel connection to the respectivetransistors. These electric components have to be inspected immediatelyafter they are assembled and subsequently at certain periodic timeintervals because of deterioration due to usage. It has been customaryto remove each electric component from the inverter circuit and inspectthe removed electric component with a dedicated instrument to determinewhether the electric component is of acceptable quality or not.

Heretofore, an electrolytic capacitor has been inspected as follows: Theelectrostatic capacitance and equivalent series resistance of theelectrolytic capacitor are measured by an LCR meter, for example, andthe electrolytic capacitor is determined as to its quality from themeasured electrostatic capacitance and equivalent series resistance. Inthe inspection process using the LCR meter, a voltage across theelectrolytic capacitor is measured while a small alternating current ofseveral mA is passing through the electrolytic capacitor, and theelectrostatic capacitance and equivalent series resistance of theelectrolytic capacitor are calculated based on the measured voltage. Theelectrolytic capacitor is determined as to its quality by checking ifthe calculated electrostatic capacitance and equivalent seriesresistance fall in respective predetermined ranges or not.

Another inspection process is disclosed in Japanese laid-open patentpublication No. 5-215800, for example. In the disclosed inspectionprocess, an electrolytic capacitor to be inspected is charged through aresistor, and a charging time required from the time when theelectrolytic capacitor starts being charged until it is charged to apredetermined voltage thereacross is measured. The measured chargingtime is compared with a reference time which is consumed until a voltageacross a normal electrolytic capacitor reaches the predetermined voltagewhen the normal electrolytic capacitor is charged. If the measuredcharging time is shorter than the reference time, then the electrolyticcapacitor which has been inspected is determined as being deteriorated.

Transistors for use in inverter circuits are required to be checked forsaturated voltage vs. emitter current characteristics (V_(CE) -I_(C)characteristics) between the emitter and the collector at both a normaltemperature and a predetermined elevated temperature in order to keepreliability of the inverter circuits. To determine whether a transistorof an inverter circuit is acceptable or not, it has been theconventional practice to disconnect the transistor from the invertercircuit, plot the V_(CE) -I_(C) characteristic curve of the transistorwith a curve tracer, and check if the plotted V_(CE) -I_(C)characteristic curve falls within a predetermined range from a referenceV_(CE) -I_(C) characteristic curve for the transistor. The V_(CE) -I_(C)characteristics are measured when the transistor junction is at a normaltemperature, e.g., 25° C., and a predetermined elevated temperature,e.g., 125° C. The inspected transistor is accepted if both the V_(CE)-I_(C) characteristics measured at these temperatures fall within thepredetermined range from reference V_(CE) -I_(C) characteristics.

According to the above conventional inspecting process of determining anelectrolytic capacitor by calculating the electrostatic capacitance andequivalent series resistance of the electrolytic capacitor which aremeasured by the LCR meter, the voltage applied to the transistor is lowas the current flowing therethrough is small. If an electrolyticcapacitor used as a smoothing capacitor in an inverter circuit isinspected, then since conditions in which the electrolytic capacitor isinspected are greatly different from those in which it is actually usedand the electrolytic capacitor is not inspected under the conditions inwhich it is actually used, the result of the inspecting process cannotbe used as being obtained under the conditions in which it is actuallyused. Another problem is that the electrolytic capacitor to be testedhas to be removed from the inverter circuit.

The equivalent series resistance of an electrolytic capacitor isconsidered as being important because it is responsible for the heatingof the electrolytic capacitor and hence greatly affects the service lifeof the electrolytic capacitor. In the inspection procedure in which ameasured charging time is compared with a reference time, the equivalentseries resistance of an electrolytic capacitor being inspected cannot bemeasured or calculated as it is only possible in the inspectionprocedure to determine the electrostatic capacitance of the inspectedelectrolytic capacitor as being reduced a certain quantity from theelectrostatic capacitance of the reference electrolytic capacitor.

The above conventional process for inspecting a transistor isproblematic in that a transistor to be inspected has to be disconnectedfrom the inverter circuit in question, and hence the inspecting processis tedious and time-consuming if the inverter circuit is in actual use.The process is time-consuming because the temperature needs to beincreased from the normal temperature (25° C.) to the elevatedtemperature (125° C.). The process requires a dedicated instrument suchas a curve tracer or the like, and involves a large number of steps forinspecting transistors on a mass-production line.

As described above, electric components for inverter circuits areindividually inspected in the conventional inspecting processes. Theconventional inspecting processes require a long period of time untilthe inspection of an electric component is finished because it isnecessary to detach the electric component from the inverter circuit,inspect the electric component, and thereafter connect the electriccomponent that is found acceptable back in the inverter circuit.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide anapparatus for inspecting an electric component in an inverter circuiteasily and highly accurately under conditions close to those in which itis actually used, while the electric component is being connected in theinverter circuit.

According to the present invention, the above objects can be achieved bya method of inspecting a transistor in an inverter circuit, while thetransistor is being connected in the inverter circuit, including thesteps of turning on only the transistor to be inspected which is beingconnected in the inverter circuit; supplying a predetermined collectorcurrent to the transistor from a power supply until a junctiontemperature of the transistor reaches a predetermined temperature; anddetermining the transistor as acceptable if a difference between acollector-to-emitter voltage of the transistor when the predeterminedcollector current is supplied from the power supply to the transistorand a collector-to-emitter voltage of the transistor when the junctiontemperature of the transistor reaches the predetermined temperaturefalls within a predetermined range.

The above objects also can be achieved by a method of inspecting anelectrolytic capacitor in an inverter circuit including the steps ofturning off switching elements included in the inverter circuit andsupplying a current from a DC power supply through a resistor to chargethe electrolytic capacitor; measuring a voltage across the electrolyticcapacitor while the electrolytic capacitor is being charged and a timeperiod from the beginning of charging the electrolytic capacitor until atime at which a voltage across the electrolytic capacitor attains apredetermined voltage; determining an electrostatic capacitance of theelectrolytic capacitor based on the measured time period measured and aresistance value of the resistor; discharging the charged electrolyticcapacitor; measuring a discharging voltage across the electrolyticcapacitor and a discharging current flowing from the electrolyticcapacitor while the electrolytic capacitor is being discharged; anddetermining an equivalent series resistance of the electrolyticcapacitor based on the voltage across the electrolytic capacitor at thebeginning of discharging the electrolytic capacitor, the determinedelectrostatic capacitance of the electrolytic capacitor, the determineddischarging voltage across the electrolytic capacitor and the determineddischarging current flowing from the electrolytic capacitor.

The electric components of the inverter circuit can be inspected whilebeing connected in the inverter circuit, rather than being disconnectedfrom the inverter circuit.

The above and other objects, features, and advantages of the presentinvention will become apparent from the following description when takenin conjunction with the accompanying drawings which illustrate preferredembodiments of the present invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an apparatus for inspecting an electriccomponent in an inverter circuit according to the present invention;

FIG. 2 is a block diagram of a controller in the apparatus shown in FIG.1;

FIG. 3 is a circuit diagram of an inverter circuit composed of electriccomponents that can be tested by the apparatus shown in FIG. 1;

FIG. 4 is a block diagram of an inspecting arrangement in which theapparatus shown in FIG. 1 is employed to determine whether anelectrolytic capacitor is acceptable or not;

FIG. 5 is a flowchart of an operation sequence of the inspectingarrangement shown in FIG. 4 for calculating an electrostatic capacitanceto determine whether the electrolytic capacitor is acceptable or not;

FIG. 6 is an equivalent circuit diagram of an electrolytic capacitor;

FIG. 7 is a flowchart of an operation sequence for calculating theequivalent series resistance of the electrolytic capacitor to determinewhether the electrolytic capacitor is acceptable or not with the circuitarrangement shown in FIG. 4;

FIG. 8 is an equivalent circuit diagram illustrative of the calculationof the equivalent series resistance of the electrolytic capacitor withthe circuit arrangement shown in FIG. 4;

FIG. 9 is a diagram showing how a voltage across the electrolyticcapacitor varies at the time of the calculation of the equivalent seriesresistance of the electrolytic capacitor with the circuit arrangementshown in FIG. 4;

FIG. 10 is a diagram showing how a discharged current from theelectrolytic capacitor varies at the time of the calculation of theequivalent series resistance of the electrolytic capacitor with thecircuit arrangement shown in FIG. 4;

FIG. 11 is a diagram showing how the equivalent series resistance of theelectrolytic capacitor varies with respect to time with the circuitarrangement shown in FIG. 4;

FIG. 12 is a diagram showing how the equivalent series resistance of theelectrolytic capacitor varies with respect to the discharge current withthe circuit arrangement shown in FIG. 4;

FIG. 13 is a block diagram of another inspecting arrangement in whichthe apparatus shown in FIG. 1 is employed to determine whether anelectrolytic capacitor is acceptable or not;

FIG. 14 is a block diagram of an inspecting arrangement in which theapparatus shown in FIG. 1 is employed to determine whether a transistoris acceptable or not;

FIG. 15 is a flowchart of an operation sequence of the inspectingarrangement shown in FIG. 14 to determine whether the transistor isacceptable or not;

FIG. 16A is a diagram showing the waveform of a collector current in theoperation sequence of the inspecting arrangement shown in FIG. 14 todetermine whether the transistor is acceptable or not;

FIG. 16B is a diagram showing a change in a collector-to-emitter voltagein the operation sequence of the inspecting arrangement shown in FIG. 14to determine whether the transistor is acceptable or not;

FIG. 16C is a diagram showing the relationship between the collectorcurrent and the collector-to-emitter voltage in the operation sequenceof the inspecting arrangement shown in FIG. 14 to determine whether thetransistor is acceptable or not;

FIG. 17 is a block diagram of an inspecting arrangement in which theapparatus shown in FIG. 1 is employed to determine whether a diode isacceptable or not; and

FIG. 18 is a flowchart of an operation sequence of the inspectingarrangement shown in FIG. 17 to determine whether the diode isacceptable or not.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows in block form an apparatus for inspecting an electriccomponent in an inverter circuit according to the present invention, theinverter circuit being also shown in FIG. 1.

The apparatus, generally designated by the reference numeral 10, has aDC power supply 14 for supplying a direct current to an inverter circuit12 which is composed of electric components to be inspected, a voltagedetector 16 for measuring a voltage at a desired location in theinverter circuit 12, a current detector 18 for detecting a current at adesired location in the inverter circuit 12, and a switching circuit 24having a matrix of switches or switching contacts for switching betweena location to which the direct current is supplied from the DC powersupply 14 and locations which are measured by the voltage detector 16and the current detector 18.

The apparatus 10 also includes a controller 26 for applying drivesignals to the inverter circuit 12 through an inverter circuit controlunit 21, the DC power supply 14, and the switching circuit 24, reading avoltage outputted from the voltage detector 16 and a current outputtedfrom the current detector 18, and determining whether each of theelectric components of the inverter circuit 12 is acceptable or not, anda display unit 28 connected to the controller 26 for displayinginformation thereon.

The controller 26 is shown in block form in FIG. 2.

The controller 26 includes a central processing unit (CPU) 32, arandom-access memory (RAM) 34 for temporarily storing results ofcalculations carried out by the CPU 32, a read-only memory (ROM) 36 forstoring a program according to which the controller 26 controls theapparatus 10, an interface (I/F) 38 with the inverter circuit controlunit 21, an I/F 40 with the DC power supply 14, and an I/F 42 with theswitching circuit 24.

The controller 26 also includes an analog-to-digital (A/D) converter 44for converting a detected output signal from the voltage detector 16into a digital signal, an A/D converter 46 for converting a detectedoutput signal from the current detector 18 into a digital signal, anelectrostatic capacitance determining circuit 48 for determining whetheran electrolytic capacitor connected in the inverter circuit 12 isacceptable or not based on the electrostatic capacitance of theelectrolytic capacitor, an equivalent series resistance determiningcircuit 50 for determining whether an electrolytic capacitor connectedin the inverter circuit 12 is acceptable or not based on the equivalentseries resistance R_(ESR) of electrolytic capacitor, a transistordetermining circuit 52 for determining whether a transistor connected inthe inverter circuit 12 is acceptable or not based on the collectorcurrent I_(C) vs. collector-to-emitter voltage V_(CE) characteristiccurve (V_(CE) -I_(C) curve) of the transistor, a diode determiningcircuit 54 for determining whether a diode connected in the invertercircuit 12 is acceptable or not, and a charging time measuring circuit56 for measuring a charging time in which an electrolytic capacitor ischarged.

FIG. 3 shows an electric circuit of the inverter circuit 12.

The inverter circuit 12 comprises an electrolytic capacitor C, aplurality of transistors Q₁ ˜Q₆, and a plurality of diodes D₁ ˜D₆connected between the collectors and emitters of the transistors Q₁ ˜Q₆,respectively.

The electrolytic capacitor C, the transistors Q₁ ˜Q₆, and the diodes D₁˜D₆ of the inverter circuit 12 are electric components to be inspectedby the apparatus 10.

Now, various inspecting arrangements for determining whether theseelectric components are acceptable or not will be described below.

First, an inspecting arrangement for inspecting electrolytic capacitor Cto determine whether it is acceptable or not will be described below.

FIG. 4 shows such an inspecting arrangement in which the apparatus shownin FIG. 1 is employed to determine whether the electrolytic capacitor Cis acceptable or not. The inverter circuit 12 is also shown in FIG. 4.

As shown in FIG. 4, an inverter circuit control unit 21 is connected tothe inverter circuit 12 for turning on and off the transistors Q₁ ˜Q₆ atpredetermined times when receiving an output signal from the controller26.

The transistors Q₁, Q₂ have a common junction "a", the transistors Q₃,Q₄ have a common junction "b", and the transistors Q₅, Q₆ have a commonjunction "c". These common junctions "a", "b", "c" are connectedrespectively to winding ends of a three-phase AC motor (not shown), forexample. The transistors Q₁ ˜Q₆ are turned on and off by the invertercircuit control unit 21 to convert a DC voltage from the DC power supply14 into AC voltages to energize the three-phase AC motor.

The controller 26 includes an electrostatic capacitance measurementindicating key 81 and an equivalent series resistance measurementindicating key 82. The switching circuit 24 has a matrix of switches orswitching contacts 101˜116 controllable by an output signal from thecontroller 26 for forming charging and discharging paths for supplying acurrent from the DC power supply 14 directly or through a resistor 64and the switching circuit 24 to the electrolytic capacitor C to chargethe electrolytic capacitor C and also discharging the electrolyticcapacitor C through a current limiting resistor 63 and a transistor 62.The apparatus 10 has a driver 61 controlled by an output signal from thecontroller 26 to energize the transistor 62 to discharge theelectrolytic capacitor C.

The resistor 64 has a resistance which is greater than the internalresistance of the DC power supply 14 and the equivalent seriesresistance of the electrolytic capacitor C. The current detector 18comprises, for example, a current transformer inserted in a current pathfor detecting a current flowing through the current path, and aconverter for converting an output signal from the current transformerinto a signal of a predetermined level. The voltage detector 16 isarranged to detect the voltage across the electrolytic capacitor C,convert the detected voltage into a voltage of a predetermined level,and output the converted voltage.

The controller 26, or more specifically the CPU 32, the electrostaticcapacitance determining circuit 48, the equivalent series resistancedetermining circuit 50, and the charging time measuring circuit 56jointly function as a switch driving control circuit 261, a clockcircuit 262, calculating/determining circuits 263, 265, and a chargingcontrol circuit 264.

The switch driving control circuit 261 turns on and off the switchingcontacts 101˜116, controls the driver 61, and turns on and off thetransistors Q₁ ˜Q₆ through the inverter circuit control unit 21.

The clock circuit 262 is responsive to an output signal from theelectrostatic capacitance measurement indicating key 81 for operatingthe switching circuit 24 under the control of the switch driving controlcircuit 261 to supply a current from the DC power supply 14 through theresistor 64 to the electrolytic capacitor C. The clock circuit 262 alsoreads a voltage across the electrolytic capacitor C, which is detectedby the voltage detector 16, through the A/D converter 44 from the timewhen the clock circuit 262 has operated the switching circuit 24, andmeasures time until the voltage across the electrolytic capacitor Creaches 63.21% of an output voltage E₁ of the DC power supply 14.

The calculating/determining circuit 263 calculates an electrostaticcapacitance C_(T) of the electrolytic capacitor C from a resistance R₁of the resistor 64 and a charging time τ measured by the clock circuit262, and determines whether the calculated electrostatic capacitanceC_(T) falls within a predetermined range or not.

The charging control circuit 264 is responsive to an output signal fromthe equivalent series resistance measurement indicating key foroperating the switching circuit 24 under the control of the switchdriving control circuit 261 to supply a current from the DC power supply14 to the electrolytic capacitor C. The charging control circuit 264keeps the switching circuit 24 in the operated condition until theoutput voltage from the voltage detector 16 becomes stable, i.e.,remains unchanged, from the time when the clock circuit 262 has operatedthe switching circuit 24.

The calculating/determining circuit 265 controls the switch drivingcontrol circuit 261 to cause the driver 61 to turn on the transistor 62from the time when the charging of the electrolytic capacitor C underthe control of the charging control circuit 264 is finished. Thecalculating/determining circuit 265 also controls the switch drivingcontrol circuit 261 to operate the switching circuit 24 to cut off thecurrent from the DC power supply 14 to the electrolytic capacitor C, andthen controls the switch driving control circuit 261 to operate theswitching circuit 24 to discharge the electrolytic capacitor C. When thedischarging of the electrolytic capacitor C is finished, thecalculating/determining circuit 265 calculates an equivalent seriesresistance from the voltage across the electrolytic capacitor C and adischarged current detected by the current detector 18, and determineswhether the calculated equivalent series resistance falls within apredetermined range or not.

The display unit 28 displays information indicative of whether theelectrolytic capacitor C is acceptable or not based on output signalsfrom the calculating/determining circuits 263, 265.

A process of calculating the electrostatic capacitance of theelectrolytic capacitor C will be described below with reference to FIG.5.

Measurement of the electrostatic capacitance of the electrolyticcapacitor C is started when the electrostatic capacitance measurementindicating key 81 is pressed. When the electrostatic capacitancemeasurement indicating key 81 is pressed, the switch driving controlcircuit 261 produces an output signal to control the inverter circuitcontrol unit 21 to turn off all the transistors Q₁ ˜Q₆. Thecalculating/determining circuit 263 controls the switch driving controlcircuit 261 to turn on only the switching contacts 101, 103, 109, 113,114 of the switching circuit 24, for thereby supplying a current fromthe DC power supply 14 through the resistor 64 to charge theelectrolytic capacitor C in a step S1. The electrolytic capacitor C nowstarts being charged through the resistor 64 in a step S2. When theelectrolytic capacitor C starts being charged, the clock circuit 262starts measuring time, and reads a voltage E_(CT) across theelectrolytic capacitor C, which is detected by the voltage detector 16,in a step S3. The output voltage from the voltage detector 16 isrepresentative of the voltage E_(CT) across the electrolytic capacitorC, which progressively increases depending on a time constant based onthe resistance R₁ of the resistor 64 and the electrostatic capacitanceC_(T) of the electrolytic capacitor C. The electrolytic capacitor C iscontinuously charged until the voltage E_(CT) across the electrolyticcapacitor C, read from the voltage detector 16, reaches 0.6321E₁ in astep S4.

If the voltage E_(CT) across the electrolytic capacitor C reaches E_(CT)=0.6321E₁ in the step S4, then the time which has elapsed from the startof the charging, i.e., the charging time τ, is read in a step S5. Then,the electrostatic capacitance C_(T) =τ/R₁ is calculated in a step S6.

The step S6 is followed by a step S7 which determines whether theelectrostatic capacitance C_(T) falls within a preset range or not in astep S7. If the electrostatic capacitance C_(T) falls within the presetrange in the step S7, then the display unit 28 displays informationindicating that the electrolytic capacitor C is acceptable in a step S8.Conversely, if the electrostatic capacitance C_(T) does not fall withinthe preset range in the step S7, then the display unit 28 displaysinformation indicating that the electrolytic capacitor C is rejected ina step S9.

The voltage E_(CT) across the electrolytic capacitor C is generallyrepresented by E_(CT) =E₁ (1-e⁻(1/CR₁.sup.)) where τ=CR₁. When t=τ, thevoltage E_(CT) across the electrolytic capacitor C is E_(CT) =E₁(1-e⁻¹)≈0.6321E₁. Therefore, in the step S6, the electrostaticcapacitance can be calculated according to C_(T) =τ/R₁ from the ratio ofthe charging time τ until the voltage E_(CT) reaches E_(CT) ≈0.6321E₁ tothe resistance R₁ of the resistor 64.

By setting the voltage of the DC power supply 14 and the resistance R₁of the resistor 64 as described above, the electrolytic capacitor C canbe supplied with a current having a value close to that under actualconditions of use. Therefore, the electrostatic capacitance can bemeasured and calculated under conditions close to those in which theelectrolytic capacitor C is actually used.

A process of calculating the equivalent series resistance of theelectrolytic capacitor C will be described below.

As shown in FIG. 6, an equivalent circuit of the electrolytic capacitorC is represented by a series-connected circuit of a capacitor having anelectrostatic capacitance C_(T) and a resistor having a resistanceR_(ESR), which is called an equivalent series resistance. When theequivalent series resistance R_(ESR) increases, the temperature of theelectrolytic capacitor C rises, affecting the service life thereof, asdescribed above.

Calculations of the equivalent series resistance will be described belowwith reference to FIG. 7.

Measurement of the equivalent series resistance is started when theequivalent series resistance measurement indicating key 82 is pressed.When the equivalent series resistance measurement indicating key 82 ispressed, the switch driving control circuit 261 produces an outputsignal to control the inverter circuit control unit 21 to turn off allthe transistors Q₁ ˜Q₆. The charging control circuit 264 and the switchdriving control circuit 261 control the switching circuit 24 to turn ononly the switching contacts 101, 103, 109, 113, 114 in a step S11. Theelectrolytic capacitor C is now charged by a current from the DC powersupply 14, and continuously charged until the voltage across theelectrolytic capacitor C detected by the voltage detector 16 remainsunchanged in a step 512. Since the electrolytic capacitor C is chargeduntil the voltage across the electrolytic capacitor C remains unchangedin the step S12, the electrolytic capacitor C is fully charged at theend of the step S12.

After the step S12, the switch driving control circuit 261 controls theswitching circuit 24 to turn on only the switching contacts 113, 114,and the driver 61 turns on the transistor 62 in a step S13. Under thiscondition, the application of the voltage from the DC power supply 14 tothe electrolytic capacitor C is cut off, whereupon the electrolyticcapacitor C is discharged through the current limiting resistor 63 andthe transistor 62 in a step S14. A discharged current I_(d) from theelectrolytic capacitor C flows through the current limiting resistor 63and the transistor 62, is detected by the current detector 18, and readthrough the A/D converter 46. A voltage V_(C) across the electrolyticcapacitor C is detected by the voltage detector 16 in a step S15.

The step S15 is followed by a step S16 in which thecalculating/determining circuit 265 calculates an equivalent seriesresistance R_(ESR) from the discharged current I_(d) and the voltageV_(c). The calculation of the equivalent series resistance R_(ESR) willbe described later on. After the step S16, the calculating/determiningcircuit 265 determines whether the discharged current I_(d) is zero ornot in a step S17. The steps S14 through S16 are repeated until thedischarged current I_(d) becomes zero. Then, the calculating/determiningcircuit 265 determines whether the equivalent series resistance R_(ESR)falls within a preset range or not in a step S18. If the equivalentseries resistance R_(ESR) falls within the preset range in the step S18,then the display unit 28 displays information indicating that theelectrolytic capacitor C is acceptable in a step S19. Conversely, if theequivalent series resistance R_(ESR) does not fall within the presetrange in the step S18, then the display unit 28 displays informationindicating that the electrolytic capacitor C is rejected in a step S20.

The calculation of the equivalent series resistance R_(ESR) in the stepS16 will be described below.

FIG. 8 shows an equivalent circuit of the electrolytic capacitor C whileit is being discharged. In FIG. 8, a resistance R_(tr) represents thesum of the resistance of the transistor 62 when it is turned on and theresistance of the current limiting resistor 63. In the followingdescription, Q_(d) represents a charge remaining in the electrolyticcapacitor C while it is being discharged, and V_(ESR) represents avoltage drop across the electrolytic capacitor C due to its equivalentseries resistance R_(ESR). When the charging of the electrolyticcapacitor C is finished, i.e., immediately before the electrolyticcapacitor C starts being discharged, a voltage across the electrolyticcapacitor C which is detected by the voltage detector 16 is indicated byV_(CT). At this time, no discharged current I_(d) flows.

Since the electrostatic capacitance of the electrolytic capacitor C isC_(T) and the voltage across the electrolytic capacitor C immediatelybefore it starts to be discharged is V_(CT), the charge Q stored in theelectrolytic capacitor C immediately before it starts to be dischargedis expressed by:

    Q=C.sub.T ·V.sub.CT                               (1).

When the electrolytic capacitor C starts being discharged from a timet=0 (FIG. 9), a voltage V_(C) across the electrolytic capacitor C whichis detected by the voltage detector 16 is progressively lowered as shownin FIG. 9, and the discharged current I_(d) is also progressivelyreduced as shown in FIG. 10.

The charge Q_(d) remaining in the electrolytic capacitor C while it isbeing discharged is expressed by the following equation (2):

    Q.sub.d (t)=Q-∫I.sub.d (t)dt                          (2).

By dividing both sides of the equation (2) by the electrostaticcapacitance C_(T), the voltage V_(d) is obtained according the equation(3) below. ##EQU1##

The voltage V_(d) (t) given according to the equation (3) is a voltageacross an ideal electrolytic capacitor C whose equivalent seriesresistance is zero while it is being discharged.

A voltage V_(c) (t) across the electrolytic capacitor C which isdetected by the voltage detector 16 while the electrolytic capacitor Cis being discharged contains a voltage drop developed due to theequivalent series resistance, the voltage drop V_(ESR) developed due tothe equivalent series resistance is expressed according to the equation(4) below.

    V.sub.ESR (t)=V.sub.d (t)-V.sub.c (t)                      (4).

Therefore, the equivalent series resistance R_(ESR) is given by thefollowing equation (5):

    R.sub.ESR =V.sub.ESR /I.sub.d                              (5).

As can be seen from the above analysis, the calculation indicated by thefollowing equation (6) is carried out in the step S16 to determine theequivalent series resistance R_(ESR) :

    {V.sub.CT -(1/C.sub.T)∫I.sub.d (t)dt-V.sub.c (t)}/I.sub.d (t)(6).

As a result, the equivalent series resistance R_(ESR) with respect tothe discharging time is given as shown in FIG. 11, and the equivalentseries resistance R_(ESR) with respect to the discharged current isgiven as shown in FIG. 12.

FIG. 13 is a block diagram of another inspecting arrangement in whichthe apparatus shown in FIG. 1 is employed to determine whether anelectrolytic capacitor is acceptable or not. FIG. 13 also shows aninverter circuit.

The inspecting arrangement shown in FIG. 13 is similar to the inspectingarrangement shown in FIG. 4 except as follows: Part of the transistorsQ₁ ˜Q₆ forms a discharging path for the electrolytic capacitor C. Thecurrent limiting resistor 63 is arranged such that it is connected bythe switching contacts 115, 116 under the control of the switch drivingcontrol circuit 261 when the electrolytic capacitor C is discharged. Theelectrolytic capacitor C is discharged through the transistors Q₁, Q₄rather than the driver 61 and the transistor 62, and a current flowingthrough the common junction "a" is detected by the current detector 18.

Since the detecting end of the current detector 18 is composed of acurrent transformer, it is easy to change locations where the currentdetector 18 is inserted for detecting the current.

A process of calculating and determining the electrostatic capacitanceof the electrolytic capacitor C and a process of charging theelectrolytic capacitor C in calculating and determining the equivalent:series resistance of the electrolytic capacitor C in the inspectingarrangement shown in FIG. 13 are the same as those in the inspectingarrangement shown in FIG. 4, and hence will not be described below.

In FIG. 13, a process of discharging the electrolytic capacitor C incalculating and determining the equivalent series resistance of theelectrolytic capacitor C, i.e., the steps S13 and S14 shown in FIG. 7,is different from that in the inspecting arrangement shown in FIG. 4.The other steps are the same as those shown in FIG. 7. Therefore, thesteps S13 and S14 for the inspecting arrangement shown in FIG. 13 willbe described below.

Following the step S12, the inverter circuit control unit 21 turns ononly the transistor Q₁, and the switch driving control circuit 261 turnson only the switching contacts 105, 111, 113, 114, 115, 116.

With these switching contacts turned on, a voltage across theelectrolytic capacitor C immediately before it starts being dischargedand a voltage across the electrolytic capacitor C while it is beingdischarged are detected by the voltage detector 16 through the switchingcontacts 113, 114. By turning on the transistor Q₄, the electrolyticcapacitor C is discharged through the transistor Q₁, the switchingcontacts 105, 115, the current limiting resistor 63, the switchingcontacts 116, 111, and the transistor Q₄. The discharged current I_(d)from the electrolytic capacitor C is detected by the current detector18. Therefore, as with the arrangement shown in FIG. 4, the steps S14through S16 are repeated until the discharged current I_(d) becomeszero, for calculating the equivalent series resistance R_(ESR).

Although the transistors Q₁, Q₄ are turned on in the arrangement shownin FIG. 13, the transistor Q₄ may first be turned on and then thetransistor Q₁ may be turned on to discharge the electrolytic capacitorC, or the transistors Q₂, Q₃ rather than the transistors Q₁, Q₄ may beturned on, and the switching contacts 106, 110 rather than the switchingcontacts 105, 111 may be turned on. Alternatively, the transistors Q₃,Q₆ may be turned on, or the transistors Q₅, Q₄ may be turned on.

In the inspecting arrangements shown in FIGS. 4 and 13, the invertercircuit control unit 21 is controlled by the output signal from theswitch driving control circuit 261 to control the turning-on and -off ofthe transistors Q₁ through Q₆ for inspecting the electrolytic capacitorC. However, the turning-on and -off of the transistors Q₁ through Q₆ maybe controlled directly by the output signal from the switch drivingcontrol circuit 261 for inspecting the electrolytic capacitor C.

As described above, in determining whether the electrolytic capacitor isacceptable or not, the electrolytic capacitor can be charged underconditions close to those in which it is actually used by selecting thevoltage of the DC power supply and the resistance of the resistor, andhence the electrostatic capacitance of the electrolytic capacitor can becalculated under conditions close to those in which it is actually used.The voltage across the electrolytic capacitor can easily be measured,and only the measurement of the voltage across the electrolyticcapacitor and the charging time is sufficient for the calculation of theelectrostatic capacitance. The electrolytic capacitor can be inspectedeasily with high accuracy while it is being connected in the invertercircuit.

While the charged electrolytic capacitor is being discharged, thevoltage across the electrolytic capacitor is measured, and thedischarged current from the electrolytic capacitor is also measured. Theequivalent series resistance of the electrolytic capacitor is calculatedbased on the voltage across the electrolytic capacitor at the time itstarts being discharged, the calculated electrostatic capacitance, themeasured discharged current, and the measured voltage across theelectrolytic capacitor. The electrolytic capacitor can thus be inspectedbased on both the electrostatic capacitance and the equivalent seriesresistance.

For calculating the equivalent series resistance, the electrolyticcapacitor is charged until the voltage thereacross remains unchanged.Consequently, the electrolytic capacitor is discharged from the fullycharged condition, allowing various data to be measured and calculatedhighly accurately.

The equivalent series resistance is obtained under conditions close tothose in which the electrolytic capacitor is actually used because theequivalent series resistance is calculated with the electrolyticcapacitor charged under conditions close to those in which it isactually used.

The equivalent series resistance can be calculated by measuring thevoltage across the electrolytic capacitor while it is being dischargedand the discharged current from the electrolytic capacitor. Since thevoltage across the electrolytic capacitor and the discharged currenttherefrom can easily be measured, the electrolytic capacitor can beinspected easily with high accuracy while it is being connected in theinverter circuit.

The discharged current is not excessive as the electrolytic capacitor isdischarged through the current limiting resistor.

The discharge current is measured by the current detector which includesa current transformer at the detecting end. Accordingly, the measuringpoint can easily be moved, and the inverter circuit does not need to bedisconnected.

FIG. 14 shows in block form a inspecting arrangement in which theapparatus shown in FIG. 1 is employed to determine whether a transistoris acceptable or not, FIG. 14 also showing an inverter circuit.

A process of determining whether transistors Q₁ through Q₆ of aninverter circuit 12 are acceptable or not by measuring V_(CE) -I_(C)curves of the transistors Q₁ through Q₆ will be described below.

FIG. 15 shows an operation sequence of the inspecting arrangement shownin FIG. 14 to determine whether the transistors Q₁ through Q₆ areacceptable or not by measuring the V_(CE) -I_(C) curves thereof.

In FIG. 14, a switching circuit 24 has the same switching contacts asthose of FIG. 4, and hence those switching contacts are omitted fromillustration. A DC power supply 14, a voltage detector 16, a currentdetector 18, and a controller 26 are identical to those shown in FIG. 2.

In response to a command for determining whether the transistors Q₁through Q₆ are acceptable or not, the CPU 32 controls the switchingcircuit 24 through the I/F 42 to charge the electrolytic capacitor Cwith a current from the DC power supply 14 for a predetermined period.After elapse of the predetermined period, the charging of theelectrolytic capacitor C is stopped, and the CPU 32 outputs a switchingsignal to the switching circuit 24 to apply the voltage from the chargedelectrolytic capacitor C between the collector and emitter of atransistor to be inspected in a step S21. Based on the applied switchingsignal, the switching contacts of the switching circuit 24 are operatedto connect a positive (+) terminal of the DC power supply 14 through theresistor 64 to a terminal P of the inverter circuit 12, connect thevoltage detector 16 between the collector and emitter of the transistorQ₁, and connect the current detector 18 to detect a collector currentI_(C) from the transistor Q₁.

Then, the CPU 32 outputs an energization start signal to the DC powersupply 14 to supply a charging current from the DC power supply 14through the resistor 64 to charge the electrolytic capacitor C of theinverter circuit 12 for a predetermined period in a step S22. Whenreceiving the output signal from the controller 26, the invertor-circuitcontrol unit 21 applies a drive signal to the bases of transistors Q₁,Q₄, for example, of the inverter circuit 12 to render conductive thetransistors Q₁, Q₄ between their collector and emitter in a step S23.The charged electrolytic capacitor C is now discharged through thetransistors Q₁, Q₄.

The voltage detector 16 detects a collector-to-emitter voltage V_(CE) ofthe transistor Q₁, and the collector-to-emitter voltage V_(CE) isconverted by the A/D converter 44 (see FIG. 2) into a digital valuewhich is supplied to the transistor determining circuit 52. The currentdetector 18 detects a collector current I_(C) from the transistor Q₁,and the collector current I_(C) is converted by the A/D converter 46into a digital value which is also supplied to the transistordetermining circuit 52 in a step S24.

The collector-to-emitter voltage V_(CE) and the collector current I_(C)vary as the electrolytic capacitor C is discharged. Thecollector-to-emitter voltage V_(CE) and the collector current I_(C) aremeasured at certain intervals of time while they are varying.

The transistor determining circuit 52 generates a V_(CE) -I_(C) curvefrom the collector-to-emitter voltage V_(CE) and the collector currentI_(C) that are measured in a step S25, and determines whether thegenerated V_(CE) -I_(C) curve falls in a preset range having apredetermined width which is stored in the transistor determiningcircuit 52 in a step S26. The result of determination is supplied to theCPU 32.

If the V_(CE) -I_(C) curve falls within the preset range, then the CPU32 outputs a signal to control the display unit 28 to displayinformation indicating that the transistor Q₁ is acceptable in a stepS27. If the V_(CE) -I_(C) curve does not fall within the preset range,then the CPU 32 outputs a signal to control the display unit 28 todisplay information indicating that the transistor Q₁ is rejected in astep S28.

The steps S21 through S28 are repeated until the V_(CE) -I_(C) curves ofall the transistors Q₁ through Q₆ are determined in a step S29.

Another embodiment of a process of determining whether a transistor isacceptable or not will be described below.

The CPU 32 outputs a switching signal through the I/F 42 to theswitching circuit 24 to operate the switching contacts of the switchingcircuit 24. The positive (+) terminal of the DC power supply 14 isconnected directly to the terminal P of the inverter circuit 12, thevoltage detector 16 is connected between the collector and emitter ofthe transistor Q₁, the current detector 18 is connected to detect acollector current I_(C) from the transistor Q₁. In this embodiment, thecommon junction "a" is connected to a negative (-) terminal of the DCpower supply 14 to allow the current detector 18 to detect the collectorcurrent I_(C) from the transistor Q₁.

Then, the invertor circuit control unit 21 applies a drive signal, dueto the output signal from the controller 26, to the base of transistorQ₁ of the inverter circuit 12 to render conductive the transistor Q₁between the collector and emitter thereof. A current supplied from theDC power supply 14 passes only through the transistor Q₁ based on anenergization start signal outputted from the CPU 32 to the DC powersupply 14.

The junction temperature of the transistor Q₁ reaches TJ(O) of 25° C.,for example, when the current supplied from the DC power supply 14 tothe transistor Q₁ reaches I_(C) after it has started being supplied. TheDC power supply 14 is controlled such that after the supplied currentmeasured by the current detector 18 has reached I_(C), the currentsupplied from the DC power supply 14 is cut off when the junctiontemperature of the transistor Q₁ reaches Ti(t) of 125° C., for example.

When the current starts being supplied from the DC power supply 14 tothe transistor Q₁, the current flowing through the collector of thetransistor Q₁ progressively increases, and is then maintained at thevalue I_(C). The current I_(C), which is measured by the currentdetector 18, flows from the collector to the emitter of the transistorQ₁. The collector current has a waveform as shown in FIG. 16A. In FIG.16A, the vertical axis represents the current detected by the currentdetector 18, and the horizontal axis represents the time measured fromthe instant the current starts being supplied from the DC power supply14. An emitter-to-collector saturated voltage V_(CE) (sat) of thetransistor Q₁ at the time the current measured by the current detector18 reaches the value I_(C) is measured by the voltage detector 16. Theemitter-to-collector saturated voltage V_(CE) (sat) will hereinafter bereferred to as an emitter-to-collector voltage V_(CE) or simply avoltage V_(CE), whose waveform is shown in FIG. 16B.

When the collector current reaches the value I_(C) due to the currentsupplied from the DC power supply 14, the junction temperature of thetransistor Q₁ is still TJ(O) of 25° C., for example. The voltagemeasured by the voltage detector 16 at the time the collector currentreaches the value I_(C) is read. The voltage measured at this time isrepresented by V_(CE) (O).

Upon elapse of a predetermined period after the collector current hasreached the value I_(C), i.e., upon elapse of a period of time until thejunction temperature of the transistor Q₁ reaches Tj(t) of 125° C., forexample, the current supplied from the DC power supply 14 is cut off. Asa result, the current flowing from the DC power supply 14 to thecollector of the transistor Q₁ is cut off. The voltage measured by thevoltage detector at the time when the current supplied from the DC powersupply 14 is cut off is read, the measured voltage being represented byV_(CE) (t). The voltage V_(CE) of the transistor Q₁ after the currenthas started to be supplied from the DC power supply 14 until the currentsupplied from the DC power supply 14 is cut off has a waveform as shownin FIG. 16B.

The period of time consumed from the time when the current has startedto be supplied from the DC power supply 14 until the collector currentreaches the value I_(C) is equal to a period of time until the junctiontemperature of the transistor Q₁ reaches TJ(O) of 25° C., for example,and is indicated as an initial period in FIG. 16B. The period of timeconsumed from the time when the collector current has reached the valueI_(C) until the current supplied from the DC power supply 14 is cut off,i.e., the period of time in which the collector current is maintained atthe value I_(C), is equal to a period of time required for the junctiontemperature of the transistor Q₁ to reach Tj(t) of 125° C., for example,from TJ(O) of 25° C., for example, and is indicated as a heating periodin FIG. 16B. In the heating period, the junction temperature of thetransistor Q₁ increases with the supplied current. The relationshipshown in FIG. 16B may be represented by the relationship between thevoltage V_(CE) and the collector current as shown in FIG. 16C. In FIG.16C, the collector current increases from the initial period up to thevalue I_(C), and is maintained at the value I_(C) during the heatingperiod until the heating period ends. If the difference between thevoltage V_(CE) (O) at the time the collector current reaches the valueI_(C) and the voltage V_(CE) (t) at the time the current supplied fromthe DC power supply 14 is cut off, i.e., the voltage V_(CE) (t) at thetime the junction temperature of the transistor Q₁ reaches Tj(t) of 125°C., for example, is represented by ΔV_(CE), then the transistor Q₁ isdetermined as normal when the ΔV_(CE) {=V_(CE) (O)-V_(CE) (t)} fallswithin a predetermined range.

Generally, the thermal resistance Zth(t) of a diode is given by themanufacturer of the diode, and is defined according to the equation (7)given below. ##EQU2## where P is the electric energy consumed by thediode, and Tjd(O), Tjd(t), Tad are the junction temperature (°K.) at thetime the supplied current reaches a predetermined value after it hasstarted being supplied, the junction temperature (°K.) at the time thepredetermined current is continuously supplied for a predeterminedperiod, and the ambient temperature (°K.) of the diode, respectively.

From the equation (7) can be determined the electric energy P that is tobe consumed by the diode to reach the junction temperature Tjd(t). Anelectric energy required to reach a target junction temperature can thusbe determined. In the above example, a period of time in which to supplythe current to reach the target junction temperature of 125° C. can bedetermined.

The junction temperature Tjd and a forward voltage Vf are related toeach other as follows: ##EQU3## where n is the emission coefficient, kis the Boltzmann's constant, q is the electric charge of an electron, Ifis the forward current, and Is is the saturated current.

Therefore, if forward voltages at the time the junction temperature isTjd(t) and Tjd(O), respectively, are indicated by Vf(t) and Tv(O),respectively then the difference ΔVf={Vf(t)-Vf(O)} therebetween isexpressed as follows: ##EQU4##

It is known that the quality of a diode can be determined from ΔVf withrespect to Tjd(t) and Tjd(O). Specifically, the diode is determined asacceptable if DVf falls in a predetermined range.

It is known that a forward voltage drop correlated to the junctiontemperature can also be utilized for the determination of a transistoras well as a diode. A current I_(C) from the DC power supply 14 can beset, and a time at which the current supplied from the DC power supply14 is to be cut off, i.e., a period of time for which the current I_(C)is to be maintained, can also be set. It is also known that the qualityof a transistor can be evaluated using ΔV_(CE) in the same manner as adiode. Therefore, when ΔV_(CE) falls within a predetermined range, thenthe transistor Q₁ is determined as acceptable, and ΔV_(CE) does not fallwithin a predetermined range, then the transistor Q₁ is determined asrejected.

When a single transistor was measured by a curve tracer, thecharacteristic curve with respect to ΔV_(CE) {=V_(CE) (t)-V_(CE) (O)}and I_(C) at the time a current which is of the same waveform and valueas FIG. 16A was supplied was identical to that shown in FIG. 16C.Acceptable transistors have ΔV_(CE) falling in a predetermined range,and rejected transistors have ΔV_(CE) falling outside of thepredetermined range. This supports the above statement that a forwardvoltage drop correlated to the junction temperature can also be utilizedfor the determination of a transistor.

The period of time required to determine the transistor Q₁ in the abovefashion is short because the transistor Q₁ can be determined while it isbeing connected in the inverter circuit.

In the above embodiment, the junction temperature Tj(O) is described asbeing 25° C. However, it may be an ambient temperature at the time ofinspection. In this case, a current I_(C) is set, a period of time untilthe junction temperature of the transistor Q₁ reaches Tj(t) of 125° C.,for example, is determined, and the current I_(C) is maintained for thedetermined period of time.

In the above description, the transistor Q₁ has been determined withrespect to its quality. However, it can readily be understood that anyof the other transistors Q₂ through Q₆ can also be determined whilebeing connected in the inverter circuit 12. To inspect any of thesetransistors Q₂ through Q₆, the common junction between the currentdetector 18 and the voltage detector 16 is connected to the collector ofthe transistor to be inspected, a current from the DC power supply 14 issupplied to the collector of the transistor to be inspected, acollector-to-emitter voltage V_(CE) of the transistor to be inspected ismeasured, and a voltage according to a drive signal is applied to thebase of the transistor to be inspected to operate only the transistor tobe inspected.

The heating period in which the collector current is maintained at thevalue I_(C) is about one second if the current value I_(C) is 400 A.Therefore, the current I_(C) and the voltage V_(CE) should preferably bedetected by simultaneous observation on a two-channel oscillograph or atwo-channel digitizer rather than the current detector 18 and thevoltage detector 16.

As described above, inasmuch as the transistors Q₁ through Q₆ can beinspected for their quality while they are being connected in theinverter circuit 12, the time and the number of steps required to checkthe transistors Q₁ through Q₆ are relatively small because it is notnecessary to disconnect the transistors Q₁ through Q₆ from the invertercircuit 12 and connect them again back in the inverter circuit 12.

Quality determination of diodes D₁ through D₆ that are coupled ininverse parallel connection between the collector and emitter of therespective transistors Q₁ through Q₆ will be described below withreference to FIGS. 17 and 18.

FIG. 17 shows in block form a inspecting arrangement in which theapparatus shown in FIG. 1 is employed to determine whether a diode isacceptable or not, and FIG. 18 shows an operation sequence of theinspecting arrangement shown in FIG. 17 to determine whether the diodeis acceptable or not.

In FIG. 17, a switching circuit 24 has the same switching contacts asthose of FIG. 4, and hence those switching contacts are omitted fromillustration. A DC power supply 14, a voltage detector 16, a currentdetector 18, and a controller 26 are identical to those shown in FIG. 2.

The CPU 32 outputs a switching signal through the I/F 42 to theswitching circuit 24, and the switching contacts of the switchingcircuit 24 are operated in response to the switching signal in a stepS41. When the switching contacts of the switching circuit 24 areoperated, an electrolytic capacitor 79 is connected across the DC powersupply 14, the voltage detector 16 is connected between the collectorand emitter of the transistor Q₁, and the current detector 18 isconnected to detect a forward current I_(f) of the diode D₁.

The inverter circuit control unit 21 receiving the output signal fromthe controller 26 now outputs a de-energization signal to thetransistors Q₁ through Q₆, thereby rendering non-conductive thetransistors Q₁ through Q₆ between their collector and emitter. The CPU32 applies an energization start signal to the DC power supply 14. Theswitching contact 101 is closed and the switching contact 105 is opened.In response to the energization start signal, the DC power supply 14supplies a charging current to charge the electrolytic capacitor 79 in astep S42.

Thereafter, the CPU 32 outputs a switching signal to open the switchingcontact 101 and close the switching contact 105. The electrolyticcapacitor 79 is now discharged through the diode D₁ in a step S43.

A voltage E_(f) developed across the diode D₁ while the electrolyticcapacitor 79 is being discharged is detected by the voltage detector 16,and converted by the A/D converter 44 into a digital signal which isoutputted to the diode determining circuit 54 in a step S44. The forwardcurrent I_(f) flowing through the diode D₁ while the electrolyticcapacitor 79 is being discharged is detected by the current detector 18,and converted by the A/D converter 46 into a digital signal which isalso outputted to the diode determining circuit 54 in the step S44.

The diode determining circuit 54 determines whether the forward currentI_(f) at the voltage E_(f) falls within a preset range stored in thediode determining circuit 54 in a step S45. The result of determinationis outputted to the CPU 32. If the forward current I_(f) falls in thepreset range, then the CPU 32 outputs a signal to control the displayunit 28 to display information indicating that the diode D₁ isacceptable in a step S46. If the forward current I_(f) does not fallwithin the preset range, then the CPU 32 outputs a signal to control thedisplay unit 28 to display information indicating that the diode D₁ isrejected in a step S47.

The steps S41 through S47 are repeated until all the diodes D₁ throughD₆ are determined in a step S48.

According to the present invention, as described above, the switchingcircuit 24 has a matrix of switching contacts, which are operable by aswitching signal from the controller 26 to establish a desired electriccircuit for successively determining the quality of the electrolyticcapacitor C, the transistors Q₁ through Q₆, the diodes D₁ through D₆which are connected in the inverter circuit 12 that is unitized.

Since the electrolytic capacitor C or the electrolytic capacitor 49charged by the DC power supply 14 is used as a DC supply source, amaximum output current from the DC power supply 14 may be reduced, andhence the DC power supply 14 may be of a small size.

The apparatus for inspecting an electric component in an invertercircuit according to the present invention can inspect an electriccomponent within a reduced time because the electric component remainsconnected in the inverter circuit, but is not disconnected from theinverter circuit, when it is inspected.

Although certain preferred embodiments of the present invention has beenshown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

What is claimed is:
 1. A method of inspecting an electrolytic capacitorin an inverter circuit, the inverter circuit including a plurality oftransistor switching elements therein for converting a direct currentinto an alternating current, the method comprising the steps of:a)turning off the plurality of transistor switching elements included inthe inverter circuit, and thereafter supplying a current from a DC powersupply through a resistor to charge the electrolytic capacitor; b)measuring a voltage across the electrolytic capacitor while theelectrolytic capacitor is being charged and measuring a time period fromthe beginning of charging the electrolytic capacitor until a time atwhich a voltage across the electrolytic capacitor attains apredetermined voltage; c) determining an electrostatic capacitance ofthe electrolytic capacitor based on the time period measured in saidstep b) and a resistance value of the resistor; d) discharging theelectrolytic capacitor which has been charged; e) measuring adischarging voltage across the electrolytic capacitor and a dischargingcurrent flowing from the electrolytic capacitor while the electrolyticcapacitor is being discharged; and f) determining an equivalent seriesresistance of the electrolytic capacitor based on the voltage across theelectrolytic capacitor at the beginning of discharging the electrolyticcapacitor, the electrostatic capacitance of the electrolytic capacitordetermined in said step c) and the discharging voltage across theelectrolytic capacitor and the discharging current flowing from theelectrolytic capacitor measured in said step e).
 2. The method accordingto claim 1, wherein the electrolytic capacitor which is discharged insaid step d) has been charged until a voltage across the electrolyticcapacitor remains unchanged.
 3. The method according to claim 1, whereinsaid step d) comprises discharging the electrolytic capacitor through acurrent limiting resistor.
 4. The method according to claim 1, whereinsaid step (e) comprises detecting the discharging current with a currenttransformer.
 5. The method according to claim 1, wherein said step d)comprises discharging the electrolytic capacitor through the pluralityof transistor switching elements included in the inverter circuit. 6.The method according to claim 5, wherein the electrolytic capacitorwhich is discharged in said step d) has been charged until a voltageacross the electrolytic capacitor remains unchanged.
 7. The methodaccording to claim 5, wherein said step d) comprises discharging theelectrolytic capacitor through a current limiting resistor.
 8. A methodof inspecting an electrolytic capacitor of an inverter circuit having aplurality of transistor switching elements therein for converting adirect current into an alternating current, while the electrolyticcapacitor is connected in the inverter circuit, comprising the stepsof:a) turning off the plurality of transistor switching elements; b)charging the electrolytic capacitor with a predetermined current througha resistor; c) detecting when a voltage across the electrolyticcapacitor reaches a predetermined voltage during charging; d)determining an elapsed time from when charging of the electrolyticcapacitor begins in said step b) until the predetermined voltage isdetected in said step c); e) determining an electrostatic capacitance ofthe electrolytic capacitor based on the elapsed time determined in saidstep d) and the resistance value of the resistor; and f) determiningthat the electrolytic capacitor is acceptable if the electrostaticcapacitance determined in said step e) is within a preset range.
 9. Amethod of inspecting an electrolytic capacitor of an inverter circuithaving a plurality of transistor switching elements therein forconverting a direct current into an alternating current, while theelectrolytic capacitor is connected in the inverter circuit, comprisingthe steps of:a) turning off the plurality of transistor switchingelements; b) charging the electrolytic capacitor with a predeterminedcurrent until a detected voltage across the electrolytic capacitorremains unchanged; c) discharging the electrolytic capacitor through acurrent limiting resistor; d) detecting a discharge current through thecurrent limiting resistor during discharging; e) detecting a voltageacross the electrolytic capacitor during discharging; f) determining anequivalent series resistance of the electrolytic capacitor based on thedischarge current detected in said step d) and the voltage detected insaid step e); g) determining if the discharge current of theelectrolytic capacitor is zero; h) repeating said steps c) through g)until it is determined in said step g) that the discharge current iszero; and i) determining that the electrolytic capacitor is acceptableif a most recent equivalent series resistance determined in said stepf), prior to determination in said step g) that the discharge current iszero, is within a preset range.
 10. An apparatus for inspecting anelectrolytic capacitor of an inverter circuit, the inverter circuithaving a plurality of transistor switching elements therein forconverting a direct current into an alternating current, while theelectrolytic capacitor is connected in the inverter circuit,comprising:a dc power supply for supplying a predetermined currentthrough a resistor; voltage detection means for detecting a voltageacross the electrolytic capacitor; switching means comprising a matrixof switches for coupling the predetermined current to the electrolyticcapacitor and coupling said voltage detection means across theelectrolytic capacitor; and control means, coupled to the invertercircuit, said switching means and said voltage detection means, forturning off the plurality of transistor switching elements, controllingsaid switching means to charge the electrolytic capacitor with thepredetermined current and to couple said voltage detection means to theelectrolytic capacitor to detect when a voltage across the electrolyticcapacitor reaches a predetermined voltage during charging, determiningan elapsed time from when charging of the electrolytic capacitor beginsuntil the predetermined voltage is detected, determining anelectrostatic capacitance of the electrolytic capacitor based on theelapsed time and a resistance value of said resistor and determiningthat the electrolytic capacitor is acceptable if the determinedelectrostatic capacitance is within a preset range.
 11. An apparatus forinspecting an electrolytic capacitor of an inverter circuit, theinverter circuit having a plurality of transistor switching elementstherein for converting a direct current into an alternating current,while the electrolytic capacitor is connected in the inverter circuit,comprising:control means, coupled to the inverter circuit, for turningoff the plurality of transistor switching elements; a dc power supply,coupled to said control means, for charging the electrolytic capacitorwith a predetermined current through a resistor; voltage detectionmeans, coupled to said control means, for detecting when a voltageacross the electrolytic capacitor reaches a predetermined voltage duringcharging; and clock means, coupled to said control means, fordetermining an elapsed time from when charging of the electrolyticcapacitor begins until detection of the predetermined voltage, saidcontrol means determining an electrostatic capacitance of theelectrolytic capacitor based on the elapsed time determined by saidclock means and a resistance value of said resistor and determining thatthe electrolytic capacitor is acceptable if the electrostaticcapacitance is within a preset range.
 12. An apparatus for inspecting anelectrolytic capacitor of an inverter circuit, the inverter circuithaving a plurality of transistor switching elements therein forconverting a direct current into an alternating current, while theelectrolytic capacitor is connected in the inverter circuit,comprising:a dc power supply for supplying a predetermined current forthe electrolytic capacitor; voltage detection means for detecting avoltage across the electrolytic capacitor; a current limiting resistorfor providing a current discharge path for the electrolytic capacitor;current detection means for detecting a discharge current flowingthrough said current limiting resistor; and control means, coupled tothe inverter circuit, said voltage detection means and said currentdetection means, for turning off the plurality of transistor switchingelements, charging the electrolytic capacitor with the predeterminedcurrent until the detected voltage of the electrolytic capacitor remainsunchanged, discharging the electrolytic capacitor through said currentlimiting resistor once the detected voltage of the electrolyticcapacitor remains unchanged, determining an equivalent series resistanceof the electrolytic capacitor based on a discharge current and a voltageof the electrolytic capacitor detected during discharge, and determiningthat the electrolytic capacitor is acceptable if the equivalent seriesresistance is within a preset range.
 13. The apparatus for inspecting anelectrolytic capacitor of an inverter circuit of claim 12, wherein saidcontrol means continues discharging the electrolytic capacitor until thedischarge current is zero and subsequently determines whether theelectrolytic capacitor is acceptable based on an equivalent seriesresistance determined from a most recent discharge current and voltageof the electrolytic capacitor.